As is well known in the art, the so-called unambiguous range of a pulse radar--i.e. the maximum distance from the radar station to an outlying object from which an echo of an outgoing pulse is received within a range sweep initiated by that pulse--depends on the pulse-repetition frequency (PRF), being given by c/2f.sub.p where c is the propagation speed of the carrier wave and f.sub.p is the chosen PRF. See, for example, pages 2 and 3 of the book "Introduction to Radar Systems" by Merrill i. Skolnik, second edition, McGraw-Hill Book Company, 1980. If a reflecting object (stationary clutter or a moving target) lies beyond that unambiguous range, its echo will arrive at the radar receiver only in the next-following range sweep or possibly in another sweep after that; however, echoes received more than one sweep after emission of the corresponding radar pulse will generally be so weak that they need not be taken into account. Usually, therefore, one need only consider reflections known as second-time-around echoes received in the sweep immediately adjoining the one initiated by the originating pulse.
When the PRF is chosen low enough to provide an extended unambiguous range, ambiguities in the Doppler frequencies of moving targets will arise, resulting in so-called blind speeds; see Skolnik, supra, page 139 under subchapter 4.10. The occurrence of such blind speeds can be minimized by operating the radar transmitter in a staggered-PRF mode, i.e. with periodic switchover between different pulse-repetition frequencies on consecutive sweeps or groups of sweeps (Skolnik, pages 114-117, subchapter 4.3). However, as pointed out by Skolnik (last paragraph of subchapter 4.3), the staggered-PRF mode does not enable a cancellation of second-time-around clutter echoes by conventional means. Such cancellation, the author notes, requires a constant PRF as well as phase coherence between consecutive pulses. That coherence is available when the radar transmitter is of the type shown in Skolnik's FIG. 4.5 (page 105), using a periodically triggered power amplifier in the output of a continuously operating coherent oscillator or COHO, but not when the carrier wave is pulsed by the periodic triggering of a magnetron oscillator as illustrated in Skolnik's FIG. 4.6 (page 106).
The power amplifier used in a transmitter of the first-mentioned type is generally a klystron, particularly in civilian applications such as airport-surveillance radars (ASR) as primarily contemplated for our present invention. In comparison with a magnetron oscillator, however, the use of a klystron in a radar transmitter of medium or high power entails a number of drawbacks including the generation of much higher voltage levels with consequent risk of harmful radiation, the need for liquid cooling with its attendant maintenance problems, as well as greatly increased size, weight and cost.